‘Worst crime scene I’ve ever seen’

Jurors couldn't hide their horror after being shown graphic images taken at a crime scene where a nanny is accused of killing two children in her care.

The images, taken by a crime scene detective and presented on Tuesday at the trial of Yoselyn Ortega in the State Supreme Court in Manhattan, showed blood stains in the hallway and a knife on a toy fridge, The New York Times reported.

Ms Ortega is accused of repeatedly stabbing Lucia (Lulu) Krim 6, and brother Leo, 2, leaving them to bleed to death in a bath in 2012.

Jurors saw some of what the children's mother Marina saw when she found them wounded.

The court also heard from emergency workers who were the first on the scene.

Eugene Nicholas, a paramedic and one of several emergency medical workers to testify, said he worked to pump oxygen into Lucia's chest, which had a hole in it.

"That's the worse crime scene I've ever seen, other than September 11," he told the court.

Prosecutors said she had been stabbed more than 30 times.

A second paramedic who worked on Leo said it was "probably the most horrendous scene" he had seen during his career.

The grim images come after jurors last week watched surveillance footage of the moment Ms Krim stood in the lobby of a dance studio discovering her daughter, who she called Lulu, was not at class.

She bends over her phone, frantically texting the family's nanny.

"Where's Lulu ... Donde Estas ... Where are you" she wrote.

But Ms Ortega never responded.

Marina Krim’s three children Lulu, Leo and Nessie.

In less than an hour, Ms Krim would find her children dead in the bathroom of her apartment.

Ms Krim finished her heart-wrenching testimony after about six hours on the stand in the murder trial of their former caregiver who prosecutors say grabbed two kitchen knives and butchered the two children while Ms Krim was just blocks away at the studio.

Ms Ortega has pleaded not guilty; her lawyer says she's too mentally ill to be responsible for what she did. However prosecutors claim she knew exactly what she was doing.

As Ms Krim left the courtroom, visibly exhausted and palpably angry, she turned to Ms Ortega and screamed: "You're evil! You're evil! And you like this, you like this, you're getting pleasure," she cried.

"Get me out of here. Please, get me out of here. Get me out!"

Marina Krim with her three children, from left, Nessie, Lulu and Leo.

The judge told jurors to disregard the outburst.

Ms Krim told the panel she and Ms Ortega, hired part-time as an extra pair of hands for the well-to-do stay-at-home mum, had worked out their schedule for October 25, 2012.

Ms Ortega, 55, was supposed to pick up Lulu from kindergarten and walk her to dance class then go home to put Leo down for nap while Ms Krim took her other daughter, three-year-old Nessie, to her swimming lesson.

Ms Krim showed up to Lulu's dance studio shortly before 5pm, and panicked when Lulu wasn't there. As she bends over her phone in footage, Nessie plays in a bright pink coat in the lobby. Then Ms Krim grabs her hand and walks out.

In this file photo, photographs of 6-year-old Lucia Krim and her 2-year-old brother, Leo, are displayed alongside balloons and stuffed animals outside the building where they lived. Picture: Mary Altaffer/AP

But New York has a high bar for the insanity defence, and it is rarely successful.

To win, Ms Ortega's lawyers will have to prove she didn't understand the consequences of her actions and didn't know right from wrong at the time.

Jurors also saw a booklet of photos Ms Krim made for Ms Ortega after a trip they all took to see the nanny's family in the Dominican Republic. The three children are beaming. In one, Ms Ortega holds them, smiling.

Jurors also heard a doorman place a 911 call, and Ms Krim's anguished screams in the background.

Ms Krim's husband, Kevin, is a former CNBC executive now at a start-up. They use a Facebook page to post updates on how they are doing, writing about the arrival of two new children, Felix born in 2013 and Linus in 2016.

The couple started the Lulu and Leo Fund, which aims to support innovative art programs for children. They recently posted a video message on Facebook asking that people mention the fund as their case becomes news again.

- With news.com.au

The building where two children were stabbed to death allegedly by their nanny in an Upper West Side apartment in New York. Picture: AFP/Getty Images